In general, it is understood that an electrical humidity sensor is a kind of transducer for indicating as relative humidity a change in electrical properties depending on a change in humidity condition, which is classified into two wide categories in basic principle, one of which utilizes a change in electric resistance and the other of which measures a change in electrostatic capacitance. The latter capacitance type, namely a type of humidity sensor which determines humidity by measurement of capacitance variation upon absorption of moisture into a humidity sensing element, has heretofore employed a porcelain plate or specially fabricated glass fibres or alumite sheet as a dielectric element, which is poor in sensitivity and thus has a practical problem. Furthermore, a humidity sensor has been recently proposed which utilizes a polymeric membrane as the dielectric element for functioning as a sensing part, as disclosed for example, in the Japanese Opened Application 74588/74, the Japanese Opened Application 66749/80 and the U.S. Pat. No. 3,350,941.
In general, when the dielectric element comprising the polymeric membrane is used as the humidity sensing layer, the dielectric element is excellent in responsiveness to the humidity and shows a good reproducibility. Furthermore, it has the advantage that a relationship between the humidity and the capacitance is quite linear, improving the measuring accuracy. On the other hand, however, the dielectric polymeric membrane has the disadvantage that it can be stripped off from a base plate upon a long period of exposure to an excessively dry or a high humidity condition, resulting in degradation of its humidity sensing property. In addition, the polymeric membrane has a further disadvantage in that it is susceptible to corrosion with organic solvents (such as acetone) so that it may not be used in an atmosphere of much organic solvent vapor, such as in a printing factory, paint factory or the like because of its adverse effect.
Thus, there has still been desired a practical capacitance humidity sensor in the art, which may be advantageously used in a harmful atmosphere, such as a high temperature and an organic solvent atmosphere, and is excellent in mechanical strength. For this requirement, there have been proposed, for example, "a humidity sensing element of metal oxide membrane" (the Japanese Opened Application 12497/77) and "a humidity detecting element using an anodic oxide membrane of aluminium" (the Japanese Opened Application 9595/78). Both of these utilize a technique of forming a conductive porous metal oxide membrane on a base plate of aluminum or the like by means of electrolysis or otherwise, which provides a rough surface of the element leading to an error of measurement, as well as an unsatisfactory mechanical strength of the metal oxide membrane itself, and thus cannot meet the requirement described hereinbefore.
Now it has been found out that instead of forming the membrane on the base plate through oxidation or electrolysis, a formation of a metal oxide membrane (such as metal oxide or nitride) on an insulating base plate, such as a glass plate or the like which is inert to oxidation, through the ion-plating technique of introducing the corresponding metal vapor and oxygen, nitrogen or ammonia gas into a high frequency field may provide a metal oxide membrane of extremely high strength, which may be well used in the severe environment described hereinabove. Furthermore, it has been found that a mechanical treatment of the metal oxide, such as a roughening treatment of its surface through plasma etching, may provide excellent responsiveness and reproducibility, as well as an improvement in accuracy of measurement.